Thermal therapy for diseased tissues and other conditions may be achieved through conversion of ultrasonic acoustic energy to thermal energy (heat) in or around the affected tissue or target site. The application of focused ultrasonic fields to a target zone or region of interest has been promising as it allows controlled and non-invasive heating of such regions by way of a focused or phased transducer source or array. The focal zone of such thermal therapy applicators can be in the few millimeter range, and allow heating of certain volumes of tissue without invasive surgical procedures. Such techniques also permit real time monitoring of the heated region by way of other imaging modalities such as magnetic resonance imaging (MRI).
Surgery using focused ultrasound beams has been carried out in animals and human patients for a variety of clinical conditions. Ultrasound surgery has been used to treat human brain tumors, to perform spinal commissurotomy, and to treat glaucoma. Several clinical trials have used prototype ultrasound devices to treat benign and malignant tumors of prostate, bladder, and kidney. More recently several clinical trials using diagnostic ultrasound to guide the surgery have been reported with encouraging results. Existing system generally rely on mechanical movement of a single focused transducer that produces a small focal volume resulting in long treatment times if the diseased region (e.g., tumor) has a substantial size to treat.
The potential of using phased arrays for ultrasound surgery has also been explored. To focus the beam, the applicator is constructed from an array of small transducer elements, which are independently driven. An intensity maximum is created by driving the transducer elements in such a way that the hemispherical waves emitted by each element (if approximated as a point source) are in the same phase at the desired focal point. The focusing is caused by superposition or constructive interference of the waves at the desired point, giving the ultrasonic field its highest intensity at the focus. Outside of the focal area the waves interfere more or less destructively or not coherently, thus minimizing the effect on the tissue the waves traverse prior to the focal point.
Phased arrays have been proposed for use in thermal coagulation of tissues. A concentric ring design, used to evaluate the feasibility of moving the focus in the depth direction exhibits promise for use in ultrasound imaging, hyperthermia treatment and focused ultrasound surgery.
Present systems are typically ill adapted to treat large volume treatment zones or volumes in an efficient manner due to the small focal spot of the typical therapy applicators and other considerations. This makes it more difficult to justify and adopt thermal therapy from ultrasound sources in clinical practice and also increases the cost of the treatments. Merely increasing the number of phased array elements in a therapeutic transducer array makes this technology expensive and has hindered its use in clinical systems. Thus, new methods are needed to make the treatments faster either by enhancing the focal energy delivery and/or making the electronically steerable phased arrays practical.